In modern communication systems, the information to be transmitted is coded both in the phase and in the amplitude of a signal. This means that it is possible to achieve considerably higher data transmission rates than those with pure amplitude and/or phase modulation. Examples of modulation types such as these are PSK modulation (Phase Shift Keying). These include, inter alia π/4-DQPSK, 8-DPSK or 8-PSK modulation. Quadrature amplitude modulation methods (QAM) also code the information to be transmitted both in the amplitude and in the phase. In contrast to analog amplitude or frequency modulation, the stated modulation methods are referred to as digital modulation types.
FIG. 9 shows a so-called constellation diagram for QPSK modulation. The abscissa in this case represents a first component, which is referred to as the real component I. The ordinate forms the second component, the quadrature component Q. The information to be transmitted is coded, as a function of its content, at one of the illustrated points by a value pair i, q. A value pair i, q such as this is referred to as a symbol. In the illustrated exemplary embodiment, one such symbol with the QPSK modulation type that is used codes a total of two bits of data content, specifically the bits 00, 01, 10 or 11. The amplitudes of the i and q values change over time, depending on the information to be transmitted. The amplitude of the overall signal is thus also changed. QPSK modulation is therefore referred to as a modulation type with a non-constant envelope (non-constant envelope modulation). The QPSK modulation type is used, for example, for the WCDMA/UMTS mobile radio standard. The EDGE mobile radio standard uses 8-PSK modulation, and thus codes 3 bits per symbol.
In addition to the representation of a symbol by a value pair i, q, it is possible to specify the phase φ and the amplitude r of the same symbol. The symbol which represents the data content 00 is illustrated in a corresponding form in the exemplary embodiment shown in FIG. 9. The two representations using IQ notation and rφ notation are equivalent.
In addition to I/Q modulators, polar modulators can also be used for transmission of modulated signals. While I/Q modulators process the i, q value pairs for modulation of a signal, polar modulators modulate the phase φ onto a carrier signal, and change the amplitude r. FIG. 7 shows one embodiment of a known I/Q modulator, in which the components I, Q are each supplied as digital signals to a digital/analog converter 901, which converts them to analog components and supplies them via a low-pass filter 902 to the inputs of two mixers 903. Signals with a phase shift of 90° between them are supplied to the two mixers as a local oscillator signal. After frequency conversion in the two mixers, the two signals are added, and are amplified in a power amplifier PA.
FIG. 8 shows one example of a known polar modulator. The information to be transmitted is in the form of digital data and is preprocessed in a coder circuit 95 to form the amplitude information r and the phase information φ. This information is in the form of symbol values ak, where ak includes not only the amplitude information r but also the phase information φ. The symbol values ak are supplied to a pulse former circuit 93, where they are preprocessed. The preprocessed data then has its phase value φ(k) and its amplitude value r(k) converted in the circuit 94. The phase information φ(k) is supplied to a phase locked loop PLL, and is used for the purpose of modulating the output signal from the phase locked loop appropriately for the information coded in the phase. A phase-modulated output signal φ(t) is thus produced at the output of the control loop PLL. At the same time, the amplitude information r(k) is applied to a digital/analog converter DAC, which converts the digital amplitude information r(k) to an analog signal r(t) which is a function of time. The analog amplitude modulation signal r(t) is supplied to a mixer via a low-pass filter. The phase-modulated signal is joined to the amplitude modulation signal in this mixer. The mixer uses the amplitude modulation signal r(t) to carry out amplitude modulation of the already phase-modulated signal.
The problem with this solution is the requirements for the last mixer stage. This should have a sufficiently highly linear transfer function in order to produce adequate signal quality within the wide amplitude range that is required in many mobile radio standards. Amplitude and phase distortion, which is dependent on the amplitude modulation signal r(t), can occur if the mixer transfer function is not linear. Distortion of this type is referred to as AM/AM or AM/PM distortion.
The distortion results in data errors, and changes the frequency spectrum of the emitted signal.
If the requirements are taken into account, the embodiment as illustrated in FIG. 8 requires a large amount of space for the mixer. Furthermore, a polar modulator such as this cannot be designed using new types of CMOS technologies with low supply voltages in the range from 1.5 V to 2.5 V.